Updated at 9:15 p.m. ET
WASHINGTON -- Despite increasing pressure on the Obama administration to take U.S. military action to stop the bloodbath in Syria, the two top Pentagon officials said Wednesday that's not even close to happening.
Under intense questioning, both Defense Secretary Leon Panetta and Joint Chiefs Chairman Gen. Martin Dempsey said military operations against Syria would be more challenging than the seven-month campaign against Libya.
A senior defense official also told NBC News that the president has not asked the U.S. military or the Pentagon to draw up plans for U.S. military intervention in Syria, despite media reports to the contrary.
NBC News has learned that the Pentagon has drafted what is called a "commander's estimate," which provides a long-term vision and expectations of what a military operation in Syria would look like. The officials point to Panetta's testimony before Senate Armed Services that there is no contingency plan for military operations in Syria, in part because the president has not asked for a plan.
Dempsey predicted it would take "several weeks" of U.S. airstrikes against Syria's air defenses just to be able to provide humanitarian aid to Syrian civilians, and a much longer "sustained campaign" to try to push Syrian President Bashar Assad from power.
Dempsey said the challenge "would be a hundred times more than what we experience in Libya."
Asked then if the U.S. could arm the opposition forces, Dempsey said it's problematic because there are "a hundred different" opposition groups in Syria. "A coherent core does not exit."
Panetta also warned there's a serious danger in attempting to arm the opposition. The fear, according to Panetta, is that the weapons could fall into the hands of terrorists, including "Hezbollah, Hamas and al-Qaida."
The best exchange came between McCain and Panetta. McCain, who is aggressively pushing for U.S. airstrikes to support the opposition repeatedly asked, "How much more killing" does there have to be? "How many more have to die. Ten thousand, Twenty thousand?" McCain added, "America should take the lead."
Panetta shot back, "We're working it. We can't do it today. But when we'll do it, we'll do it right."
Panetta himself revealed however that the Pentagon and U.S. military are doing "no contingency planning" for military operations in Syria. Panetta said they're waiting for that request to come from President Barack Obama.
Still, there is an underlying motive emerging for military strikes against Syria. Gen. James Mattis, who heads the U.S. Central Command, on Tuesday said that getting rid of Assad would deal the worst strategic defeat to ally Iran in more than 20 years.
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